Plasmas are ionized gases that may be created by a variety of methods. One way of creating plasmas is by using electromagnetic energy to ionize a gas. Plasmas may conduct electricity and interact with electromagnetic radiation. Acoustic sources produce pressure waves in a medium through which acoustic waves travel, such as a solid, liquid, or gas. These pressure waves may change the local density of the material through which the acoustic waves travel.
Metamaterials are manmade materials having properties not normally found in nature. For example, some metamaterials display a negative index of refraction. Many metamaterials are constructed from multiple elements of composite materials arranged in a repeating pattern or structure. The elements of a metamaterial are generally smaller than the wavelength of the phenomena a metamaterial influences. For example, a metamaterial acting as an optical wave guide may have a pattern made of elements smaller than the wavelength of light which interacts with the metamaterial.